1. Field of the Invention
This invention relates to pneumatic radial tires, and more particularly to a pneumatic radial tire for a passenger car simultaneously establishing a reduction of rolling resistance and an improvement of steering stability.
2. Description of the Related Art
Recently, needs for a reduction of fuel consumption in vehicles from a viewpoint of environmental protection have considered attempts for reducing the rolling resistance in tires. In this connection, there have hitherto been attempted various proposals for reducing the rolling resistance of the tire. An example of these proposals is a tire shown in FIG. 2.
When this tire is compared with the conventionally used tire shown in FIG. 3, a cross sectional area of a stiffener 4 having an approximately triangular shape is made small to lower a height of the stiffener. A turnup portion 3-2 of a carcass ply 3 is wound around a bead core 2 from inside toward outside is extended adjacent to a carcass ply body 3-1 from a position lower than that of the tire shown in FIG. 3. On the other hand, a cross sectional area of a rubber chafer 5 is made large and a thickness of the rubber chafer 5 is made maximum in the vicinity of an outward end of the stiffener 4 in the radial direction of the tire. The rubber chafer 5 is extended outward over the stiffener 4 in the radial direction of the tire. Moreover, a ratio of cross-sectional area in the stiffener to cross-sectional area in the rubber chafer is not more than 0.5.
A factor reducing the rolling resistance in the tire of the above structure, first is first that a rubber material having tan .delta. smaller than that in the stiffener 4 is usually used in the rubber chafer 5. That is, as the ratio of the rubber material having a large tan .delta. occupying a portion deformed during the running under a load becomes larger, the energy consumption becomes larger or the rolling resistance becomes larger. Therefore, the occupying ratio of the stiffener 4 is made small in the tire shown in FIG. 2.
Secondly, since the occupying ratio of the stiffener 4 in the tire of FIG. 2 is small, the tire rigidity in the radial direction of the tire is lowered to make the deformation of the belt end small. That is, the belt end portion has a great influence on the rolling resistance of the tire, so that the rolling resistance can effectively be reduced by controlling the deformation of the belt end portion.
However, when the occupying ratio of the stiffener is made small in the tire of FIG. 2, the rigidity of the bead portion becomes rather insufficient and hence the steering stability is degraded. For this end, it is required to take a countermeasure for supplementing such a rigidity shortage. In this connection, it has hitherto been attempted to arrange a reinforcing member comprised of metal or organic fiber cords slantly extending with respect to the circumferential direction of the tire to wind around the bead core from inside toward outside, or to arrange such a reinforcing member outside the stiffener in the axial direction of the tire. According to these conventional techniques, the circumferential rigidity is certainly improved, but the radial rigidity is simultaneously improved to lose the effect of reducing the rolling resistance. On the other hand, when the cords in the reinforcing member are arranged substantially in the radial direction without inclining with respect to the circumferential direction, the circumferential rigidity is not increased, but the radial rigidity is increased too much, so that the rolling resistance is undesirably increased and also the noise is generated.